Cold-Sprayed Al6061 Coatings: Online Spray Monitoring and Inﬂuence of Process Parameters on Coating Properties

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Article Cold-Sprayed Al6061 Coatings: Online Spray Monitoring and Inﬂuence of Process Parameters on Coating Properties

Heli Koivuluoto 1,* , Jussi Larjo 2, Danilo Marini 3, Giovanni Pulci 3 and Francesco Marra 3 1 Materials Science and Environmental Engineering, Faculty of Engineering and Natural Sciences, Tampere University, 33100 Tampere, Finland 2 Oseir Ltd., 33720 Tampere, Finland; [email protected] 3 INSTM Reference laboratory for Engineering of Surface Treatment, Department of Chemical Engineering Materials Environment, Sapienza University of Rome, 00185 Rome, Italy; [email protected] (D.M.); [email protected] (G.P.); [email protected] (F.M.) * Correspondence: heli.koivuluoto@tuni.ﬁ; Tel.: +358-40-849-0188

Received: 25 February 2020; Accepted: 1 April 2020; Published: 3 April 2020

Abstract: Process optimization and quality control are important issues in cold spraying and coating development. Because the cold spray processing is based on high kinetic energy by high particle velocities, online spray monitoring of particle inflight properties can be used as an assisting process tool. Particle velocities, their positions in the spray jet, and particle size measurements give valuable information about spraying conditions. This, in turn, improves reproducibility and reliability of coating production. This study focuses on cold spraying of Al6061 material and the connections between particle inflight properties and coating characteristics such as structures and mechanical properties. Furthermore, novel 2D velocity scan maps done with the HW CS2 online spray monitoring system are presented as an advantageous powder and spray condition controlling tool. Cold spray processing conditions were similar using different process parameters, confirmed with the online spray monitoring prior to coating production. Higher particle velocities led to higher particle deformation and thus, higher coating quality, denser structures, and improved adhesions. Also, deposition efficiency increased significantly by using higher particle velocities.

Keywords: cold spray; diagnostics; aluminum; Al6061; coatings; online monitoring; process parameters

1. Introduction Cold spraying has shown its potential for producing high-quality coatings with different industrial requirements. For example, corrosion resistance, electrical conductivity, wear resistance, functional properties, and repairing possibilities are application fields for cold-sprayed coatings. Especially, for corrosion protection and repairing possibilities, Al and Al-based alloy coatings have received a lot of interest due to their good material properties and high suitability for cold spraying as the feedstock materials [1–5]. In cold spraying, low melting point materials such as Al alloys can be sprayed without oxidation. This is because the cold spraying is the solid-state coating processing method, where coating formation is based on a high kinetic energy instead of thermal energy compared to other thermal spray technologies, e.g., arc and flame spray processes. Powder particles are accelerated with a high pressurized and preheated gas (N2, He, air) to a high velocity and they impact to a substrate surface, adhere, deform, and form a coating. [6–11] During the impacts, particles need to undergo a high plastic deformation in order to achieve high-quality coating structures. This, in turn, depends on the particle velocity. [12] Assadi et al. [12] have well summarized the influences of several factors

Coatings 2020, 10, 348; doi:10.3390/coatings10040348 www.mdpi.com/journal/coatings Coatings 2020, 10, 348 2 of 16

on the particle velocity: (1) particle velocity is higher while spraying with He compared to N2; (2) particle velocity increases with increased gas temperature; (3) higher pressure leads to higher particle velocity; (4) particle velocity can be increased with certain nozzle geometries such as when length, expansion ratio or diameter increase, particle velocity increases; (5) by increasing the spray distance to the certain maximum level, particle velocity increases and then, after maximum distance decreases; (6) particle velocity decreases if spraying angle increases; and (7) if powder feeding rate increases, particle velocity decreases. As noted, cold spray processing is influenced by several factors and they have combined effects. The process itself (low-, medium-, or high-pressure cold spray process) affects spray parameter selections, which can be used for spraying of different powder materials. These spray parameters have affected the particle inflight properties and powder deformation as well as thermal softening and adiabatic shear instabilities of impacted particles. [6–10] All factors should be taken into account while selecting the process parameters. In addition to this, powder and its properties can affect the sprayability and the deform capability during the processing as well as the spray parameter selection. Particles need to achieve certain material-dependent velocities in order to form a coating. Below the critical velocity, particles just rebound and do not adhere to the sprayed surface. Also, there is an upper velocity limit and above that, erosion starts to play a role and the number of bonded particles decreases. [12–14] Yin at al. [15] have reviewed the gas and particle interactions in cold spraying. Particle velocities has been modelled by using computational fluid dynamics (CFD) models [14,16] and experimentally investigated by using different measuring techniques such as laser-2-focus (L2F) [17], doppler picture velocimetry (DPV) [18–21], and particle imaging velocimetry (PIV) [22–25]. Also, we have demonstrated the advanced optical diagnostic method called PTV (particle tracking velocimetry) or S-PTV (sizing-particle tracking velocimetry) [26], which is based on the PIV approach. In this measurement technique, more accurate results can be investigated due to its specific telecentric optics and advance measuring algorithm. Here, individual particle velocities, particle size, and particle position in the spray jet can be analyzed and used for process optimization and the spray parameter selection. Furthermore, the process quality and especially, powder travelling properties can be controlled. Online spray monitoring can indicate if any feeding variables or nozzle condition changes occur. In addition to particle velocity measurement with diagnostic measuring devices, powder jet has been investigated with an infra-red camera [27]. Its purpose is to observe integral particle stream properties rather than properties of individual particles in cold spraying. Therefore, it can give just an additional information about the powder jet. Mean particle velocities have been reported in many studies [17,19,20,28]. Gilmore et al. [17] have investigated mean particle velocities and separate velocities of Cu particles by using the L2F technique, resulting in the smaller particles having higher velocities. Jodoin et al. [28] have studied cold spray particle velocities by modeling and connecting those particle velocities to experimental measurements with the PIV technique. They reported higher particle velocities with higher pressures and temperature used. Measured mean particle velocities were slightly higher or close to velocities modelled with CFD. In addition, shock waves of gas flow in the nozzle have been modelled, showing the dropping of the particle velocity before nozzle exit. This shock wave behavior cannot be detected with online monitoring techniques, which indicated that different analyzing techniques are supporting each other [18,20,28]. In addition, Silvello et al. [29] have analyzed several studies and depicted a process parameter, which has been described as material density divided by particle diameter times gas pressure per gas density. They have connected this to particle velocities. Particle velocity increased with increasing process parameter until reaching a certain point and then started to decrease. Also, particle velocity has been influenced by particle shape. Fukunuma et al. [19] have reported that irregular particles had higher average particle velocities compared with spherical particles. Aluminum alloys are relatively easy to spray with cold spraying, but also present challenges caused by nozzle clogging, powder agglomeration during feeding, and uneven feeding. Therefore, process controlling, and quality checking are very important and advantageous in order to avoid Coatings 2020, 10, 348 3 of 16 the problems during the spray processing. We focus on aluminum alloy Al6061 because it has high corrosion resistance and good mechanical properties and thus, it is used in many common working conditions, e.g., in aircraft and automotive industries. Al6061 is a precipitation-hardened Al-based alloy, which has Mg and Si as major alloying elements [30]. Earlier, we achieved improvements in coating quality and properties by using laser-assisted cold spraying compared to traditional medium-pressure cold spraying [2]. Structure was denser and adhesion was higher together with higher deposition efficiency. In this study, we use a high-pressure cold spray system with advanced processing conditions for producing Al6061 coatings on Al and steel substrates. This study focuses on the optimization of cold spraying of Al6061 by using novel online spray monitoring for the spray parameter selection and as the quality control tool. From the online monitoring point of view, particle velocities, particle size, and particle positions in the gas flow or gas jet are the factors that have been measured and used for the process optimization. There are multiple factors that influence the spray process, and they are summarized here. Particle-gas flow has specific features depending on the cold spray system used (e.g., high pressure vs. low pressure and axial feeding vs. radial feeding), process gas (N2, He, air), and nozzle design, geometry, and shape (e.g., round vs. rectangular shape), as well as process conditions (pressure, temperature, powder feeding) and powder properties (melting point, particle size distribution, shape and flowability). Also, these different factors influence mutually in the cold spray process. Therefore, it is crucial to develop the quality control aspects and methods together with the process development itself. This way, repeatability can be checked and at the same time, fast notification about cold spray equipment conditions can be achieved. For example, problems with uneven powder feeding, starting of nozzle clogging or wearing of the nozzle can be detected with the online spray monitoring. Importantly, fast diagnostics of powder conditions can be analyzed with particle size measurements. Sometimes, altered powders can lose their optimal properties during storage and this can be detected as particle size variations between different spray times. Particle inflight properties directly affect the coating properties and quality, and therefore, it is encouraged to study these properly during the cold spray process. Online spray monitoring was done with a laser illuminated PTV as a multi-streak detection. In this study, a sheet illumination was used with HiWatch CS2 (HW CS2) online spray monitoring equipment. Furthermore, in this study, a 2D model for particle velocity data with particle position and amounts is presented. This trigging is shown first time here and its benefits are related to showing velocity distribution and cross-dependencies. After online spray monitoring, coating properties of cold-sprayed Al6061 coatings, and effect of spray parameters on those are analyzed and connected to particle velocities.

2. Materials and Methods Aluminum alloy powder, Al6061, was used as a feedstock material (Al6061, TLS Technik, Bitterfeld-Wolfen, Germany). Composition is Al (bal-wt %), Si 0.40–0.80 wt %, Fe max. 0.7 wt %, Cu 0.15–0.40 wt %, Mn 0.15 wt %, Mg 0.8–1.2 wt %, Cr 0.04–0.35 wt %, Zn max 0.25 wt %, and Ti max. 0.15 wt %. The powder was produced by gas-atomization for cold spraying, and it had a particle size distribution of 40 + 10 µm. Particles were spherical in shape, Figure1. − Coatings were manufactured with high-pressure cold spray equipment, PCS-100 (Plasma Giken Co., Ltd., Saitama, Japan). Nitrogen was used as a process gas. Spray parameters were selected as following: pressures of 30–40 bar, temperatures of 300–400 ◦C, spray distance of 40 mm, step size of 1 mm, traverse speed of 10–20 m/min, powder feed of 1.5 rpm (roughly measured as ~10 g/min), and 4 coating layers. Converging-diverging de Laval nozzle with the outer diameter of 28 mm was used in these experiments. The nozzle was plastic, having a round shape. Coatings were sprayed using a robot (IRB 4600, ABB Ltd., Helsinki, Finland). Grit-blasted (Al2O3, Mesh40) aluminum and low-carbon steel were used as substrates. Table1 shows the test conditions used in this research for the online spray monitoring as well as for the coating production. Coatings 2020, 10, 348 4 of 16 Coatings 2019, 9, x FOR PEER REVIEW 4 of 16

Figure 1. Morphology of gas gas-atomized-atomized Al6061 powder. SEM image.

Coatings were manufacturedTable 1. Cold with spray high test-pressure conditions cold and spray prepared equipment, coatings. PCS-100 (PlasmaGiken Co., Ltd., Saitama, Japan). Nitrogen was used as a process gas. Spray parameters were selected as following:Test pressures Pressure of 30 (bar)–40 bar, temperatures Temperature (of◦C) 300– Online400 °C, Spray spray Monitoring distance of 40 mm, Coating step Code size of 1 mm, 1traverse speed of 30 10–20 m/min, powder 300 feed of 1.5 rpm (roughly X measured as ~10 g/min) - , and 4 coating2 layers. Converging 30-diverging de 350 Laval nozzle with the outer X diameter of 28 mm - was used in these3 experiments. The 30 nozzle was plastic, 400 having a round shape. X Coatings were sprayed Al30_400 using a 4 35 300 X Al35_300 robot (IRB 4600, ABB Ltd., Helsinki, Finland). Grit-blasted (Al2O3, Mesh40) aluminum and low- 5 35 350 X Al35_350 carbon6 steel were used 35 as substrates. Table 400 1 shows the test conditions X used in this research Al35_400 for the online7 spray monitoring 40 as well as for the 300coating production. X - 8 40 350 X - 9 40Table 1. Cold spray 400test conditions and prepared X coatings. Al40_400

Test Pressure (bar) Temperature (°C) Online Spray Monitoring Coating Code Novel1 online30 spray monitoring technique,300 PTV, (HiWatchX CS2, HW CS2, Oseir Ltd.,- Tampere, Finland)2 was used30 prior to spraying350 the coating samples. ParticleX velocity, position, and- size were 2 measured3 from the30 mid-point (area400 8 5 mm , 2 MP, 35 fps (GigE)).X In our previousAl30_400 study [26], × we used4 HW HR235 (Oseir Ltd., Tampere,300 Finland) online sprayX monitoring system,Al which35_300 detects similar5variables as35 HW CS2 used in this350 study. HW HR2 bases onX shadow imaging whereasAl35_350 HW CS2 monitoring6 is based35 on a sheet measurement,400 where particles areX detected by scattering.Al Both35_400 systems 7 40 300 X - are based on imaging optical setup from narrow axial cross section of the spray and pulsed diode laser 8 40 350 X - illumination of particles. 9 40 400 X Al40_400 Compared to the more advanced S-PTV technique used earlier [26], this simple PTV technique has onlyNovel coarse online particle spray sizing,monitoring but it technique, oﬀers reduced PTV, system (HiWatch size CS2, and costHW withCS2, muchOseir higherLtd., Tampere, particle measurementFinland) was used rate. prior In both to cases,spraying the the illumination coating samples. and individual Particle particlevelocity, detection position, areand based size were on a triple-pulsemeasured from scheme the mid [26].-point The pulse (area interval8 × 5 mm in2, the 2 MP, triplet 35 fps was (GigE)). 400 ns. In The our camera previous (Figure study2a) [26], itself we is lightweightused HW HR2 and (Oseir simple Ltd., to use. Tampere Figure, Finland)2b shows online particle spray imaging monitoring geometry system, used inwhich the measurements.detects similar Detailsvariables of asthe HW measurement CS2 used details in this setup study. can HW be HR2found bases from on [26 shadow]. Single imaging illuminated whereas image HW covers CS2 a spray jet cross section of (0.5 5 mm2). For this reason, precise alignment of spray nozzle with the monitoring is based on a sheet× measurement, where particles are detected by scattering. Both systems imagingare based plane on imaging is needed. optical The camera setup from control narrow software axial provides cross section automated of the image spray processing and pulsed and diode spray particlelaser illumination position and of velocityparticles. measurement, and also calculates mean values, distributions, cross-plots, and historyCompared graphs to the for variousmore advanced particle statistics.S-PTV technique In addition, used 2D earlier spray [26], cross this section simple mapping PTV technique was done (Figurehas only2c). coarse The sheetparticle translation sizing, but was it offers done withreduced triggered system burst size imaging,and cost with where much the sheethigher traverses particle themeasurement spray plume rate. during In both image cases, sequence the illumination capture. A and typical individual image sequence particle isdetection captured are using based 1 mm on /as scantriple speed-pulse producing scheme [26]. 250 The individual pulse interval images in (capture the triplet time was ~10 400 s), thus ns. The providing camera a depth(Figure resolution 2a) itself ofis 0.04lightweight mm. Total and analysis simple to time use. is Figure ~15 s. The2b shows resulting particle particle imaging data tablegeometry can be used converted in the measurements. to a 2D map of particleDetails of properties the measurement describing details the particle setup can jet velocitybe found and from density [26]. Single distributions illuminated across image the measured covers a spray jet cross section of (0.5 × 5 mm2). For this reason, precise alignment of spray nozzle with the

Coatings 2019, 9, x FOR PEER REVIEW 5 of 16 imaging plane is needed. The camera control software provides automated image processing and spray particle position and velocity measurement, and also calculates mean values, distributions, cross-plots, and history graphs for various particle statistics. In addition, 2D spray cross section mapping was done (Figure 2c). The sheet translation was done with triggered burst imaging, where the sheet traverses the spray plume during image sequence capture. A typical image sequence is captured using 1 mm/s scan speed producing 250 individual images (capture time ~10 s), thus providing a depth resolution of 0.04 mm. Total analysis time is ~15 s. The resulting particle data table Coatings 2020, 10, 348 5 of 16 can be converted to a 2D map of particle properties describing the particle jet velocity and density distributions across the measured jet cross section. 2D velocity and density scan maps were measured withjet crosstotal cross section. section 2D velocity areas of and 8 × 8 density mm2. scan maps were measured with total cross section areas of 8 8 mm2. ×

(a)

(b) (c)

FigureFigure 2. 2. OnlineOnline spray spray monitoring (a(a)) HW HW CS2 CS2 system, system, (b) particle(b) particle imaging imaging geometry, geometry, and (c )and scanning (c) scanningplume forplume 2D mapping.for 2D mapping.

PowderPowder morphology morphology was was analyzed withwith aa scanning scanning electron electron microscope microscope (SEM, (SEM, Jeol IT-500,Jeol IT-500, Tokyo, Tokyo,Japan). Japan). Coating Coating structures structures were, were, in turn, in turn, studied studied with with an opticalan optical light light microscope microscope (LM, (LM, Leica Leica DM DM2500, 2500, Wetzlar, Wetzlar, Germany) Germany) and aand field-emission a field-emission scanning scanning electron electron microscope microscope (FESEM, (FESEM, Zeiss UltraPlus, Zeiss UltraPlus,Jena, Germany). Jena, Germany). Cross sections Cross ofsections the coatings of the werecoatings done were following done following a standard a sample standard preparation sample preparationmethod, including method, polishingincluding stepspolishing with steps SiC papers with SiC (up papers to 2500), (up diamondto 2500), diamond suspensions suspensions (3 and 1 µ (3m), andand 1 µm), final polishingand final polishing with SiO 2withsuspension. SiO2 suspension. Vickers hardnessVickers hardness was measured was measured with a Vickers with a Vickers hardness hardnesstester (Matsuzawa tester (Matsuzawa MMT-X7, MMT-X7, Akita, Japan)Akita, Japan) by using by theusing load the of load 100 of g. 100 The g. resultsThe results are givenare given as an asaverage an average of ten of measurements. ten measurements. Coating Coating thicknesses thicknesses were were measured measured from thefrom coating the coating crosssections cross sectionspictured pictured with LM with and LM the and results the results are given are as given an average as an average of ten measurements. of ten measurements. AdhesionAdhesion strength strength of ofcoatings coatings was was measured measured through through a amechanical mechanical testing testing machine machine (Instron (Instron 5584,5584, Norwood, Norwood, MA, MA, USA) USA) at ata crosshead a crosshead rate rate of of1 1mm/min mm/min according according to to the the ASTM ASTM C633 C633 [31]. [31 ].Each Each testtest specimen specimen was was obtained obtained assembling assembling one one cylindri cylindricalcal coated coated substrate substrate to tothe the sand-blasted sand-blasted faces faces of of thethe loading loading fixtures fixtures by by an an adhesive adhesive bonding bonding agent agent (Polyamide-epoxy (Polyamide-epoxy FM FM 1000 1000 Adhesive Adhesive Film, Film, bond bond strength ~70 MPa); a self-aligning device was adopted for applying the tensile load to the assembly of the coating and fixtures.

3. Results and Discussion Firstly, this study focuses on online spray monitoring, and secondly, coating properties sprayed with the selected spray parameters. Particle velocity and size as well as velocity distribution together with particle densities and positions are evaluated and followed with the characterization of the coating properties such as structure, thickness, hardness, and adhesion. Coatings 2020, 10, 348 6 of 16 3.1. Particle Velocity Measurements 3.1.Firstly, Particle we Velocity present Measurements what kind of results can be collected from the measurement data with the opticalFirstly, diagnostic we present system, what HW kindCS2, used of results in this can study. be collected Examples from of the the particle measurement velocity datadistribution, with the particleoptical diagnosticsize distribution, system, particle HW CS2, velocity used indistribution this study. versus Examples particle of the position particle in velocity the spray distribution, jet, and statisticsparticle sizeof the distribution, data measured particle with velocity HW CS2 distribution system are presented versus particle in Figure position 3. This in shows the spray that jet, many and variablesstatistics can of the be dataevaluated measured simultaneously. with HW CS2 system are presented in Figure3. This shows that many variables can be evaluated simultaneously.

(a) (b)

FigureFigure 3. 3. ExampleExample of ofthe the HW HW CS2 CS2 measurement measurement data: data: (a) particle (a) particle speed speed distribution, distribution, (b) particle (b) particle size distribution,size distribution, (c) particle (c) particle speed speed distribution distribution in x-position in x-position with with fitting, ﬁtting, and and (d ()d statistics) statistics from from the the measuredmeasured data. data.

InIn this this study, study, particle particle velocities velocities sprayed sprayed with with nine nine totally totally different different combinations combinations of of pressures pressures (low:(low: 30 30 bar, bar, medium: medium: 35 35 bar, bar, and and high: high: 40 bar) 40 bar) and and temperatures temperatures (low: (low: 300 °C, 300 medium:◦C, medium: 350 °C, 350 and◦C, high:and high:400 °C) 400 were◦C) weremeasured. measured. The particle The particle velocities velocities as a function as a function of pressure of pressure and temperature and temperature are presentedare presented in Figure in Figure 4. When4. When pressure pressure and temperatur and temperaturee increase, increase, particle particle velocity velocity increases increases as stated as instated many in studies many studies[12,13,25,26,32]. [12,13,25 In,26 the,32 ].present In the stud presenty, temperature study, temperature has a higher has ainfluence higher influence on particle on particle velocity compared to pressure while spraying the Al6061 powder. Especially, when spraying with the highest temperature (400 ◦C) and the lowest pressure, particle velocities are almost at the similar level while spraying with the lowest temperature and the highest pressure. Increment of the particle velocities with medium or high pressure compared to low pressure is 4% and 6%, respectively. Similarity spraying with the highest pressure (40 bar) increment of the particle velocity with medium or high temperature compared to low temperature is 2% and 5%, respectively. The highest difference in the average particle velocities is between the lowest (30 bar, 300 ◦C) and the highest parameters Coatings 2019, 9, x FOR PEER REVIEW 7 of 16 velocity compared to pressure while spraying the Al6061 powder. Especially, when spraying with the highest temperature (400 °C) and the lowest pressure, particle velocities are almost at the similar level while spraying with the lowest temperature and the highest pressure. Increment of the particle velocities with medium or high pressure compared to low pressure is 4% and 6%, respectively. Similarity spraying with the highest pressure (40 bar) increment of the particle velocity with medium or high temperature compared to low temperature is 2% and 5%, respectively. The highest difference in the averageCoatings 2020 particle, 10, 348 velocities is between the lowest (30 bar, 300 °C) and the highest7 ofparameters 16 (40 bar, 400°C) used. Particle velocities sprayed with the highest parameters were 10.5% higher than sprayed (40with bar, the 400 lowest◦C) used. parameters. Particle velocities sprayed with the highest parameters were 10.5% higher than sprayed with the lowest parameters.

Figure 4. Particle mean velocities sprayed with different gas pressures and gas temperatures. Figure 4. Particle mean velocities sprayed with different gas pressures and gas temperatures. Meyer et al. [25] have measured particle velocities for Al with the PIV technique and the velocity dependence on the particle mass feed rates, resulting in decreased particle velocities with higher Meyerfeed et rates. al. [25] Aluminum have measured particles sprayed particle with velociti the pressurees for Al of 15with bar the or 30 PIV bar technique had average and particle the velocity dependencevelocities on the of ~450particle and mass ~540 m feed/s, respectively rates, resulting [25], using in decreased similar feed particle rates as wevelocities have used. with In higher our feed rates. Aluminumstudy, particle particles velocities sprayed are higher, with probably the pressure due to theof 15 higher bar gasor 30 temperatures bar had average and pressures particle used, velocities of ~450 anddiff erences~540 m/s, in particle respectively size and [25], different using particle similar velocity feed measuringrates as we technique have used. used In (PIV our vs. study, PTV). particle velocitiesSchmidt are higher, et al. [ 13probably] have shown due critical to the velocities higher aroundgas temperatures 620–660 m/s for and Al and pressures its alloys. used, In this differences study, in all measured velocity values are above this, and therefore, coating formed successfully with all the particle sizeselected and spray different parameters. particle velocity measuring technique used (PIV vs. PTV). Schmidt et al. [13] have shownIn addition critical to velocities traditional around particle velocity620–660 measurements, m/s for Al and 2D its velocity alloys. map In wasthis measured study, all and measured velocity valuesanalyzed. are This above novel this, measurement and therefore, setup coatin showsg the formed deviation successfully of particle positionswith all andthe particleselected spray parameters.velocities in the spray plume from the cross-sectional direction. Particle concentration in different In additionx-positions to in traditional the spray jet particle as well as velocity particle velocity measurements, range in z-positions. 2D velocity Figure map5a–c showswas measured the 2D and velocity scan maps for different gas pressures (30, 35, and 40 bar) used with the temperature of 400 ◦C, analyzed.respectively. This novel It ismeasurement clearly seen that setup velocity shows range increasedthe deviation with increased of particle pressure. positions Additionally, and particle velocitiesparticle in the positions spray plume in cross-sectional from the area cross-sectiona in the spray plumel direction. can be detected.Particle Thisconcentration is an advantageous in different x- positionsquality in the control spray tooljet toas checkwell theas particle nozzle and veloci powderty range feeding in conditions z-positions. during Figure the measurements. 5a–c shows the 2D velocity scanThe measurements maps for different were achieved gas pressures without substrate, (30, 35, andand therefore, 40 bar) bowused shock with eff theect wastemperature not analyzed. of 400 °C, respectively.However, It is bow clearly shocks seen formed that close velocity to substrate range surface increased has shown with to aff increasedect particle impactpressure. velocity Additionally, [15]. High-pressure and high-density gas generated this bow shock layer, which decreases particle velocity: particle positionsthis influence in cross-sectional is stronger with small area particlesin the spray [15,33 ].plume In this can study, be spray detected. conditions This wereis an suitable, advantageous quality controlconfirmed tool with to these check 2D velocitythe nozzle scan maps.and powder 2D velocity feeding scan maps conditions showed that during the highest the velocitiesmeasurements. The measurementsare in the middle were area achieved of the spray without jet in x and substr z directions.ate, and Similarity, therefore, Meyer bow et al. [shock25] have effect shown was not analyzed.that However, particle velocities bow shocks of Al powder formed was close the highestto subs intrate the middle surface of has the sprayshown jet into x affect and directions. particle impact velocity [15]. High-pressure and high-density gas generated this bow shock layer, which decreases particle velocity: this influence is stronger with small particles [15,33]. In this study, spray conditions were suitable, confirmed with these 2D velocity scan maps. 2D velocity scan maps showed that the highest velocities are in the middle area of the spray jet in x and z directions. Similarity, Meyer et al. [25] have shown that particle velocities of Al powder was the highest in the middle of the spray jet in x and directions.

Coatings 2020, 10, 348 8 of 16 Coatings 2019, 9, x FOR PEER REVIEW 8 of 16

(a)

(b)

(c)

Figure 5.5. 2D velocityvelocity scanscan maps. maps. Cold Cold spraying spraying of of Al6061 Al6061 powder powder with with (a) ( 30a) 30 bar bar and and 400 400◦C, °C, (b) 35(b) bar 35 andbar and 400 400◦C, °C, and and (c) ( 40c) bar40 bar and and 400 400◦C. °C. Contour Contour lines lines show show velocity. velocity. Colored Colored mapsmaps presentpresent particleparticle densities. Scan area is 8 × 88 mm mm22. . ×

Coatings 2020, 10, 348 9 of 16

3.2. Particle Size Measurements The PTV technique in HW CS2 enables measuring particle sizes for quality inspection. This method analyzes particle sizes above 20 µm for calculations but detects smaller sizes as well. It was a suitable monitoring tool for our study to verify the spray conditions and control powder quality. If particle size is changing, it could indicate agglomeration or some sticking during the powder feeding and spraying. In addition, one advantage of measuring particle sizes with diagnostics is that an amount of the measured particles is much higher compared to measurements done by image analysis from microscopic samples or with laser diffraction particle size measuring method [26]. In this study, particle sizes were in a similar level, indicating uniform powder feeding and powder conditions between several measurements and spraying. Particle position was measured by measuring the area and particles/mm2, and the particle sizes where analyzed from this data. Table2 shows the particle mean size, DV50 (cumulative 50% particle size; half of the particles have the size below this value and the half above this value), and DV90 (cumulative 90% particle size; 90% of particles have the size below this value) values when spraying the powder with the pressure of 35 bar and different temperatures (300, 350, and 400 ◦C). The maximum difference in the particle sizes sprayed with different parameters is 0.9 µm, which is kept very low by taking into account deviation coming from the particle size distribution.

Table 2. Particle sizes measured with HW CS2 using pressure of 35 bar and gas temperatures of

300–400 ◦C. Mean size, DV50, and DV90 values are presented.

Pressure (bar) Temperature (◦C) Mean Speed (m/s) Mean Size (µm) DV50 (µm) DV90 (µm) 35 300 686 29.5 35.2 45.6 35 350 707 30.1 35.5 43.4 35 400 729 30.4 36.4 43.6

It is important to measure the particle size together with particle velocities. Additionally, particle velocity is depended on the particle size; smaller particles are faster and the larger ones opposite are slower [13]. Also, as noted earlier, powder properties can be checked and noticed with particle size measurements, acting as fast quality controlling. Importance of this was noticed also by Mauer et al. [33] and in the review given by Yin et al. [15]. Particle morphology can have an influence on velocity as well. Especially, when comparing modelled and measured data, differences can be caused by morphology differences. Usually, in modelling, particles are expected to be spherical and have a certain particle size, but the practice has shown that there can be also some agglomerates, irregular or angular particles together with spherical ones [34]. Therefore, if powder is measured online from the spray jet, the particle size, powder quality, and powder feeding conditions can be checked prior spraying.

3.3. Cold-Sprayed Al6061 Coatings–Structural Properties Coatings were sprayed with five selected spray parameters. The parameters resulting in the lowest particle velocities were left out from the coating characterization since high velocity assisted the higher coating quality. High temperature with low, medium, and high pressure as well as medium pressure with low, medium, and high temperature were selected for coating production. This way, we can investigate the effect of particle velocities on coating properties. First, coating thicknesses were evaluated as an indicator for deposition efficiency. Particle velocities had a noticeable effect on the coating thicknesses, as presented in Figure6. Relative thickness was represented per one coating layer and it was normalized with the traverse speed. When spraying with high temperature, increment of coating thickness was +13% when comparing medium pressure to low pressure and +45% with high pressure compared to low pressure. The effect of temperature on the coating thickness was stronger than the effect of pressure. Coating thickness sprayed with medium pressure and medium temperature was +65% higher compared to the coating thickness sprayed with medium pressure and low temperature. Thickness increased +54% when sprayed with medium pressure and high Coatings 2020, 10, 348 10 of 16 Coatings 2019, 9, x FOR PEER REVIEW 10 of 16 mediumtemperature pressure compared and high to medium temperature pressure compared and + 154%to medium compared pressure to the and low +154% temperature. compared Higher to the lowgas temperaturetemperature. assistedHigher gas thermal temperature softening assisted of the th particles,ermal softening which, of in the turn, particles, increased which, the particlein turn, increaseddeformation the [ particle7,10,12]. deformation In addition to[7,10,12]. this, heating In addition increased to this, the heating adherence. increased Also, withthe adherence. higher pressure Also, withand temperature higher pressure of the and gas, temperature higher particle of the velocity gas, high is achieved,er particle which, velocity in turn, is achieved, increases which, the deposition in turn, increasesefficiency [the35]. deposition Therefore, efficiency it could be [35]. said thatTherefore, the deposition it could effibeciency said that increased the deposition in this study efficiency as well whileincreased comparing in this thestudy coating as well thicknesses. while comparing In all the th cases,e coating coating thicknesses. thickness increasedIn all the with cases, increasing coating thicknessspray parameters. increased Thiswith indicates increasing that spray we parameters. are in the suitable This indicates spray window that we areaare in with the suitable these selected spray windowspray parameters. area with these If temperature selected sp orray pressure parameters. is increased If temperature even more, or pressure there willis increased come the even limit more, and thereabove will this, come thickness the limit starts and to decrease, above this, as shown thickness Schmidt starts et to al. decrease, [13]. This as was shown due toSchmidt the erosion et al. by [13]. the Thisparticles was thatdue startedto the erosion to play by a role the duringparticles the that impacts started [13 to]. play a role during the impacts [13].

Figure 6. 6. RelativeRelative thicknesses thicknesses of the of thecold-sprayed cold-sprayed Al6061 Al6061 coatings coatings sprayed sprayed with different with di ﬀsprayerent parameters.spray parameters.

Cold-sprayed Al6061Al6061 coating coating structures structures sprayed spraye withd with high high temperature temperature (400 ◦ C)(400 and °C) with and diﬀ erentwith differentpressures pressures (30, 35, and(30, 4035, bar)and 40 are bar) presented are presented in Figure in Figure7. All 7. the All coatings the coatings are relatively are relatively dense dense by bymicroscopic microscopic analysis analysis and and well well adhered adhered to to the the substrate. substrate. Particle Particle velocityvelocity waswas higherhigher thanthan critical velocity for aluminum (620–660 m/s m/s [13]) [13]) and therefore,therefore, coating structures were dense, and coating was well deformed. However, by increasing pressure, higher deformation can be achieved. This was seen as a purer structure sprayed with 40 bar and 40 4000 °C◦C (Figure 77c),c), comparedcompared toto structurestructure sprayedsprayed with 30 bar and 400 ◦°CC (Figure7 7a).a). InIn thethe latterlatter case,case, somesome openopen particleparticle boundariesboundaries werewere detecteddetected due to less deformed particles and more more oxidized particle boundaries that were not destroyed during the particle impacts. Also, Also, Cavaliere Cavaliere and and Sivello Sivello [5] [5] have have concluded that that with with the nonoptimal spray conditions, adhesion of the cold-s cold-sprayedprayed Al-based coatings was weaker and the structure included more pores, which, in turn, aaffectedﬀected the fatiguefatigue resistanceresistance ofof thethe coatings.coatings. In general, high quality coatings had better fatigue resistance [[5].5].

Coatings 20202019,, 109, ,x 348 FOR PEER REVIEW 11 of 16

(a) (b)

(c)

Figure 7. Structure and interface between CS Al6061 coat coatinging and Al substrate sprayed with different diﬀerent spray parameters (a) 30 bar, 400400 ◦°C,C, (b)) 3535 bar,bar, 400400 ◦°C,C, and ((cc)) 4040bar, bar, 400°C. 400 ◦C. LM LM images. images.

In cold spraying, powder particles experience high high plastic deformation during during the the impacts. impacts. Bonding of particles and substrate, and particles together, is based on this plastic deformation and adiabatic shearshear instabilities instabilities at theat the interface. interface. Furthermore, Furthermore, during during the particle the particle impacts, impacts, coming particlescoming densifyparticles the densify previous the previous particle structuresparticle structures by hammering by hammering or tamping or tamping them. [11 them.] Sometimes, [11] Sometimes, there is athere porous is a porous top layer top in layer the in cold-sprayed the cold-sprayed coatings coatings because because the hammeringthe hammering effect effect of nextof next particles particles is missing.is missing. However, However, this this can can be be diminished diminished by by increasing increasing particle particle velocity velocity byincreasing by increasing temperature temperature and pressureand pressure [36]. [36]. Dense Dense microstructures microstructures of cold-sprayed of cold-sprayed Al6061 Al6061 coatings coatings are also are visual also invisual FESEM in FESEM images, Figureimages,8 .Figure Particles 8. Particles were highly were deformed highly deformed and only and some only open some particle open boundaries particle boundaries can be detected can be withdetected the coatingswith the sprayedcoatings withsprayed low with or medium low or medium temperatures temperatures and pressures. and pressures. Cold-sprayed Cold-sprayed Al6061 coatingAl6061 coating sprayed sprayed with the with high the temperature high temperature (400 ◦C) (400 and pressure°C) and pressure (40 bar) was(40 bar) very was dense very and dense particles and wereparticles extensively were extensively deformed. deformed. Cross-points Cross-points between between particles particles have been have highlighted been highlighted with the with circles the incircles Figure in 8Figurea–e, and 8a–e, especially, and especially, the cross-point the cross-point between between highly deformed highly deformed particles particles has been has marked been inmarked Figure in8e. Figure This illustrates8e. This illustrates high deformation high deformation and bonding, and bonding, which in which turn, in reflect turn, the reflect high-quality the high- coatingquality coating structure. structure. Here, cold-sprayed Al6061 coatings confirmed that higher particle velocity assisted the higher coating quality by means of denseness and purity. High particle velocity led to high deformation, which, in turn, led to tight bonding and removal of original powder particle oxide layers during the impacts [10]. If we compare the structures in Figure8a,e, the di fference was detectable while comparing the parts highlighted with the circles. Mean particle velocity difference between these coatings was 70 m/s (Figure4), which, in turn, had significant influence in this study.

Coatings 2020, 10, 348 12 of 16 Coatings 2019, 9, x FOR PEER REVIEW 12 of 16

(a) (b)

(e)

FigureFigure 8.8. MicrostructuresMicrostructures ofof cold-sprayed cold-sprayed Al6061 Al6061 coatings coatings sprayed sprayed with with di ﬀdifferenterent spray spray parameters parameters (a)

35(a) bar;35 bar; 300 300◦C, °C, (b) ( 35b) bar;35 bar; 350 350◦C, °C, (c) (c 30) 30 bar; bar; 400 400◦C, °C, (d ()d 35) 35 bar; bar; 400 400◦ C,°C, and and ( e(e)) 40 40 bar; bar; 400 400◦ °C.C. CirclesCircles showshow thethe cross-pointscross-points betweenbetween particles.particles. FESEMFESEM images.images.

3.4. Cold-SprayedHere, cold-sprayed Al6061 Coatings–MechanicalAl6061 coatings confirmed Properties that higher particle velocity assisted the higher coatingSpray quality parameters by means did of not denseness have detectable and purity. eﬀects High on coating particle hardness. velocity led All to cold-sprayed high deformation, Al6061 which, in turn, led to tight bonding and removal of original powder particle oxide layers during the coatings in this study had a hardness around 96 HV0.1, Figure9. They are much harder than Al6061-O fullyimpacts annealed [10]. If bulk we material compare (Brinell the structures hardness 30in [30Figure], ~40 8a,e, HV), the which difference is annealed was in detectable the temperature while comparing the parts highlighted with the circles. Mean particle velocity difference between these of 415 ◦C[37]. Typically, Al alloys are solution heat-treated in order to increase their mechanical properties.coatings was Hardness 70 m/s (Figure of Al6061-T4 4), which, bulk material,in turn, had which significant is solution influence heat-treated in this and study. naturally aged [37], is 65 of Brinell hardness [30] (74 HV) and furthermore, hardness of Al6061-T6, which is solution heat treated3.4. Cold-Sprayed and artiﬁcially Al6061 overaged Coatings–Mechanical [37], is 95 of Brinell Properties hardness [30] (111 HV). Hardness of cold-sprayed Al6061Spray coatings parameters were much did not higher have than detectable Al6061-T4 effects but on slightly coating lower hardness. than Al6061-T6,All cold-sprayed meaning Al6061 that duringcoatings the in coldthis study spray had processing, a hardness high around mechanical 96 HV properties0.1, Figure 9. can They be achievedare much withoutharder than the needAl6061- of O fully annealed bulk material (Brinell hardness 30 [30], ~40 HV), which is annealed in the temperature of 415 °C [37]. Typically, Al alloys are solution heat-treated in order to increase their

Coatings 2019, 9, x FOR PEER REVIEW 13 of 16 mechanical properties. Hardness of Al6061-T4 bulk material, which is solution heat-treated and naturally aged [37], is 65 of Brinell hardness [30] (74 HV) and furthermore, hardness of Al6061-T6, whichCoatings is2020 solution, 10, 348 heat treated and artificially overaged [37], is 95 of Brinell hardness [30] (11113 HV). of 16 Hardness of cold-sprayed Al6061 coatings were much higher than Al6061-T4 but slightly lower than Al6061-T6, meaning that during the cold spray processing, high mechanical properties can be achievedan addition without post-treatment the need suchof an as addition annealing. post-treatment Gavras et al. such [1] haveas annealing. produced Gavras Al6061 et coatings al. [1] have with producedcold spraying Al6061 by coatings using He with as the cold process spraying gas. by They using achieved He as the hardness process forgas. the They as-sprayed achieved coating hardness as forhigh the as as-sprayed 105 HV100. Hardnesscoating as was high slightly as 105 higher HV100 than. Hardness in this study.was slightly Furthermore, higher annealingthan in this increased study. Furthermore,ductility and diannealingﬀusion occurred increased at theductility particle and boundaries. diffusion occurred Similarity, at resistance the particle to fatigueboundaries. crack Similarity,growth was resistance increased to with fatigue annealing crack [1growth]. In our was study, increased the coating with hardnessesannealing were[1]. In at our a similar study, level the coatingas they werehardnesses in the previouswere at study,a similar where level the as coatings they were were in sprayedthe previous with laser-assistedstudy, where cold the spraying.coatings wereThis indicatessprayed with possible laser-assisted recovering cold already spraying. during This the sprayingindicates withpossible the higherrecovering process already temperature during the [2]. sprayingThe coating with hardness the higher was process ~106 HVtemperature0.3, which [2]. was The slightly coating higher hardness than was the hardness~106 HV0.3 in, which this study. was slightlyEarlier, thehigher medium-pressure than the hardness cold in spraythis study. system Earlier, with the medium-pressure pressure of 27.5 bar cold and spray the temperaturesystem with theof 350 pressure◦C was of used 27.5 as bar the processand the parameters. temperature Unfortunately, of 350 °C was we didused not as measure the process particle parameters. velocities Unfortunately,in the previous we study, did butnot wemeasure expect particle that they velocities were lower in the than previous in this study, study but due we to expect the lower that spray they wereparameter lower values than in used. this study due to the lower spray parameter values used.

Figure 9. VickersVickers hardness (HV 0.10.1) )and and standard standard deviations deviations of of the the cold-sprayed Al6061 coatings sprayed with different diﬀerent spray parameters.

Adhesion of the cold-sprayed Al6061 coatings was tested with the tensile pull tests.tests. Substrate material together with the process parameters influe inﬂuencednced on the bonding. Earlier, Earlier, it was noticed that coating adhesion was was higher on on soft metal substrate substrate compared compared to to harder harder substrate substrate material material [2,38]. [2,38]. Similarly here, adhesion strength of the cold-sprayed Al6061 coatings were higher on the Al substrate substrate (~35 MPa) MPa) compared compared to to those on the low carbon steel substrate (~20 MPa), Figure 10.10. In In all all cases, cases, fracture occurred in the coating-substrate interface, indicating adhesive failure. It seemed seemed that certain certain pressure level is is needed needed for for good good bonding. bonding. In In this this study, study, pressure pressure of of35 35bar bar or 40 or bar 40bar was was needed needed for sprayingfor spraying Al6061 Al6061 material material with with cold cold spraying, spraying, which which meant meant that thatparticle particle velocity velocity was washigher higher than than 700 m/s.700 mHigher/s. Higher particle particle velocity velocity was was beneficial beneﬁcial for for the the good good adhesion adhesion and and cohesion. cohesion. Bonding Bonding was improved withwith highhigh particleparticle velocities velocities due due to to the the higher higher deformation deformation and and thermal thermal softening softening caused caused by byincreased increased impact impact temperature temperature [5]. [5]. Adhesion of the cold-sprayed coatings can be improved by spray parameterparameter optimization, or with advanced substrate surface pre-treatments, e.g. e.g.,, laser texturing [39] [39] or by using assisting spray processes, e.g., laser-assisted cold spraying [2,3] [2,3] or by heat treatmentstreatments asas post-treatmentpost-treatment [[40].40]. While spraying Al alloy material by using cold spray proc processingessing with the pressure of 30 bar and heating of 500 °C,◦C, Kromer Kromer et al. [37] [37] achieved achieved adhesion bond strengths for Al alloy coatings on grit-blasted, and two laser textured Al alloy substrates as 20, 40, and 50 MPa, respectively. It shows that with the best spray parameter combination in our study, we can achieveachieve the coating adhesion close to the level that can be achieved with laser texturing. Our Our previous previous results results [2] [2] with a medium-pressure cold spray system operated at at 27.5 bar and 350 °◦C achieved bond strengths strengths of of 27 and 9 MPa for Al and steel substrates, respectively. Furthermore, the same system with an additional laser assistance produced coatings with bond strengths of 48 MPa for Al and 35 MPa for Al and steel substrates, respectively. Coatings 2019, 9, x FOR PEER REVIEW 14 of 16 Coatings 2020, 10, 348 14 of 16 substrates, respectively. Furthermore, the same system with an additional laser assistance produced coatings with bond strengths of 48 MPa for Al and 35 MPa for Al and steel substrates, respectively. The results ofof thethe presentpresent study study in in Figure Figure 10 10 show show that that even even higher higher adhesion adhesion can can be achievedbe achieved with with the theadvanced advanced cold cold spray spray system system using usin theg optimizedthe optimized process process parameters. parameters.

Figure 10. AdhesionAdhesion strengths strengths of of the the cold-sprayed cold-sprayed Al Al60616061 coatings coatings sprayed sprayed with with different diﬀerent spray parameters on grit-blasted Al and steel substrates.

4. Conclusions Conclusions Process optimization for cold-sprayed Al6061 powder was done by using online monitoring technique togethertogether with with coating coating characterization. characterization As. already As already known, known, particle particle velocity hasvelocity an important has an importantrole in cold role spraying in coldfor spraying successful for successful and high-performance and high-performance coating production. coating production. Therefore, Therefore, here we hereanalyzed we analyzed the influence the influence of spray of parametersspray parameters on the on particle the particle mean mean velocity velocity and and its connectionsits connections to tothe the coating coating structure structure and and properties. properties. Particle Particle velocity velocity increasedincreased withwith increasedincreased pressurepressure and gas temperature used. In In addition, addition, novel novel 2D 2D velocity velocity scan scan maps maps with the information about particle amounts connectedconnected to to their their velocities velocities in thein spraythe sp jet.ray This jet. supported This supported quality controllingquality controlling and indicated and indicatedmore efficiently more efficiently the powder the and powder spray and conditions. spray conditions. In this study, In this powder study, conditions powder conditions were controlled were withcontrolled 2D velocity with 2D scan velocity maps scan and maps particle and sizeparticle measurements, size measurements, and they and supported they supported optimal optimal spray sprayconditions conditions from the from technical the pointtechnical of view. point The of nozzle view. worked The wellnozzle and worked powder well agglomeration and powder was agglomerationnot observed, indicating was not observed, appropriate indicati cold-sprayng appropriate processing. cold-spray processing. Cold-sprayed Al6061 Al6061 coatings coatings were were relatively relatively dense, dense, but butby using by using higher higher particle particle velocities velocities using higherusing higher process process parameters parameters resulted resulted in even in even dens denserer and and more more deformed deformed coating coating structures structures were achieved. In In this this study, study, relatively relatively high high process process pa parametersrameters (high (high pressures: pressures: 30–40 30–40 bar, bar,and andhigh high gas temperaturegas temperature for forAl alloys: Al alloys: 300–400 300–400 °C)◦ C)were were used used due due to to the the wide wide spray spray parameter possibilities available in the high-pressure cold spray equipment.equipment. Process Process parameters parameters did did not not affect affect the coating hardness inin thisthis study.study. HardnessesHardnesses were were in in the the same same level, level, ~96 ~96 HV. HV. On On the the other other hand, hand, adhesion adhesion of the of thecoating coating was slightlywas slightly influenced influenced by process by process parameters. parameters. Both high Both pressure high and pressure high gas and temperature high gas wastemperature needed forwas higher needed coating for higher adhesion, coating resulting adhesion from, resulting the higher from particle the higher velocities, particle and velocities, therefore, andhigher therefore, impact conditionshigher impact during conditions the spraying. during Next, the spraying. our research Next, will our focus research on cold will spraying focus on of thickcold sprayingstructures of utilizing thick structures the knowledge utilizing achieved the knowledge in this study. achieved For in this, this accurate study. For quality this, control accurate by quality online controlmonitoring by online plays keymonitoring role together plays withkey role optimized together process with conditionsoptimized usedprocess while conditions going towards used while cold goingspray additivetowards manufacturing.cold spray additive manufacturing.

Author Contributions: Conceptualization, investigation and writing—original draft, H.K.; investigation and editing, J.L.; investigation, editingediting D.M.,D.M., G.P.,G.P., and and F.M. F.M. All authors have read and agreed to the published version of the manuscript. Funding: Funding: This research received no external funding. Funding: This research received no external funding. Acknowledgments: Authors would like to thank Mr. Jarkko Lehti of Tampere University, Thermal Spray Center Finland (TSCF), Tampere, Finland for spraying the coatings and M.Sc. Jarmo Laakso of Tampere University,

Coatings 2020, 10, 348 15 of 16

Acknowledgments: Authors would like to thank Jarkko Lehti of Tampere University, Thermal Spray Center Finland (TSCF), Tampere, Finland for spraying the coatings and Jarmo Laakso of Tampere University, Tampere, Finland for SEM characterization of the powder. This work made use of Tampere Microscope Center facilities at Tampere University, Tampere, Finland. Also, The Scientific Advisory Board for Defense (MATINE), Finland is acknowledged. Conflicts of Interest: The authors declare no conflicts of interest.

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